title: Simultaneous multislice multiband parallel radiofrequency excitation with independent slice-specific transmit B1 homogenization.
abstract: OBJECTIVE
  To develop a new parallel transmit (pTx) pulse design for simultaneous multiband (MB) excitation in order to tackle simultaneously the problems of transmit B1 (B1+) inhomogeneity and total radiofrequency (RF) power, so as to allow for optimal RF excitation when using MB pulses for slice acceleration for high and ultrahigh field MRI.
  
  
  METHODS
  With the proposed approach, each of the bands that are simultaneously excited is subject to a band-specific set of B1 complex shim weights. The method was validated in the human brain at 7T using a 16-channel pTx system and was compared to conventional MB pulses operating in the circularly polarized (CP) mode. Further numerical simulations based on measured B1 maps were conducted.
  
  
  RESULTS
  The new method improved B1+ homogeneity by 60% when keeping the total RF power constant and reduced total RF power by 72% when keeping the excitation fidelity constant, as compared to the conventional CP mode.
  
  
  CONCLUSIONS
  A new pTx pulse design formalism is introduced targeting slice-specific B1+ homogenization in MB excitation while constraining total RF power. These pulses lead to significantly improved slice-wise B1+ uniformity and/or largely reduced total RF power, as compared to the conventionally employed MB pulses applied in the CP mode.
authors.affiliation: University of Minnesota, Center for Magnetic Resonance Research, Minneapolis, Minnesota, USA.
authors.forename: Xiaoping
authors.initials: X
authors.lastname: Wu
authors.forename: Sebastian
authors.initials: S
authors.lastname: Schmitter
authors.forename: Edward J
authors.initials: EJ
authors.lastname: Auerbach
authors.forename: Steen
authors.initials: S
authors.lastname: Moeller
authors.forename: Kâmil
authors.initials: K
authors.lastname: Uğurbil
authors.forename: Pierre-François
authors.initials: PF
authors.lastname: Van de Moortele
mesh_terms: D001921:Brain / Q000502:physiology*; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods*
keywords: RF pulse design; high field MRI; parallel excitation; simultaneous multislice; transmit B1 homogenization